I alway follow my build and got nearly ALLWAY good stats.
Mostly im overfeedet and deal much dmg, even to Tanks!!!

Rarely i fail becasue i have to fight one of the Champs you dont want to fight against (Point 3)
But i still deal good dmg.

If you are allready a good Karthus player and looking for a good build, here is it.

I promise that you will get good stats...when your mates dont fail to say Miss xD

GL & HF Summoner

I run Mejai´s Soulstealer becasue u get a few kills from your AND the side lanes.
When you reach lvl 6 u should have a Mejai´s Soulstealer because of ur Ulti.
Gettting easy some charges wich makes your mid-lane easier to play.

After that buy a Rod of Ages.
You have more HP and MP and also u get a bit AP.
In 1on1 fights its advantageous when u survive longer than your enemy.
Your Passive helps alot to kill him after he killed you.

Archangerl´s Staff give you enought mana and AP and becasue of that u schould it buy after you got ur boots.

I think the rest of the items is self-explanatory.

At the start : Framing!!!
its easy to kill minions with your "Q" spell and harvest gold.
Start killing enemy Champions when ur got ur Ultimate and/or Mejai´s Soulstealer.
Keep the enemy Champs away from your minions so he cant farm. Thats very easy with your "Q" spell.

DO NOT FARM WITH "E" because it cost to much mana. U can use "E" when there a very much minions but otherwise NOT!!!

I use Teleportation and Clarity for easy and nice farming.
Clarity: I use it becasue sometimes u have to fight your enemy Champ and so you burst down your mana (sometimes in mid of a fight) its very important to have all time Mana !!
Teleportation: if you die/go shpping/gank ore whatever u can easily go back to your lane and farm/ hold it.

Here is a list of a few champs u dont want to fight.

1. Veigar: Why veigar? becasue of his ulti. His Ultimate does more dmg even more AP you have and witch my buid u got easiely over 400 ap. The Ultimate will kill you faster then you can watch. My Tipp: By other items wich lesser AP or say your team to focus him.

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2. Kassadin: Kassadins "Q" have huge range and makes you helpless. if you´re silenced... ure almost dead. You can´t hold him away from you when he reached lvl 6. My Tipp: try to haress and kill him before he gets a hight lvl.

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3. LeBlanc: Hen she reached lvl 6 she WILL burst you down. "Q" and "R" will burst over 600 HP frok you also she can silence. Tipp:try to haress and kill him before she gets a hight lvl. PLaying devensive isnt a good idea. Shes Faster and can port.

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4. ??? If you know someonewho makes you problems tell me ^^